High temperature resins are increasingly replacing metals in the fabrication of machinery parts and mechanical components. As a result, significant reductions in production and replacement costs for the machinery parts and mechanical components have been realized. To replace metals in machinery parts and mechanical components, the high temperature resins should have high resistance to mechanical wear, surface stress, and extreme temperature conditions. Additionally, the performance characteristics of the high temperature resins should equal or exceed that of the metals being replaced.
Polyimides are particularly preferable high temperature resins because of their mechanical strength, dimensional stability, thermal stability, chemical stability, flame retardance, and dielectric properties. Polyimides, such as those described in U.S. Pat. No. 3,179,614 issued to Edwards on Apr. 20, 1965, can be used in a wide variety of commercial applications. The outstanding performance characteristics of these polymers under stress and at high temperatures have made them useful in the form of bushings, seals, electrical insulators, compressor vanes and impellers, pistons and piston rings, gears, thread guides, cams, brake linings, and clutch faces.
Blending of resin polymers to improve the physical characteristics of the blend over the individual resin polymers is well known in the art. Known techniques used in processing resin polymers include dry blending, direct compression, wet granulation, melt blending, coprecipitation from solution, and spray freezing of frozen particles. Dry blending of resins is advantageous because of the absence of solvents and other liquids that may contribute to residual moisture. Dry blending is also advantageous because of its simplicity compared to other mixing methods.
It is important that a molded article have low moisture pickup because absorbed moisture can negatively affect: the dimensional stability of the molded article through, for example, hygroscopic expansion; mechanical properties such as tensile strength; electrical properties; and hydrolytic stability. Thus, it is desirable to lower the moisture pickup of a molded article without changing any of the properties of the article.
U.S. Pat. No. 4,820,781 issued to Policastro et al. on Apr. 11, 1989, discloses polyetherimide-silicone copolymer blends with a glass transition temperature of at least 190° C.
U.S. Pat. No. 4,987,197 issued to Ohta et al. on Jan. 22, 1991, discloses melt processible polyimide-aromatic polyamideimide blends that can be used in various molding applications such as injection molding, compression molding, transfer molding, and extrusion molding.
U.S. Pat. No. 5,179,153 issued to George on Jan. 12, 1993, discloses polyimide compositions containing graphite filler and polytetrafluoroethylene filler.
Accordingly, one aspect of this invention is to provide dry blended resin particulates wherein the moisture pickup of a molded article comprising the dry blended resin particulates is lower than the level expected based on the moisture pickups of the individual components of the blend. Another aspect of the invention is that these dry blended resin particulates are suitable for compression molding.